OLEDs have been proposed for use in flat panel display devices, however the light emissive materials in an OLED age over time with use. The aging rate is directly related to the cumulative current density passed through the light emitting materials. This is a problem because as the resolution of a display device increases for a given display size, the pixel sizes decrease, thereby increasing the rate of aging for a given brightness.
One approach to dealing with the aging problem, while maintaining the resolution of the display, is to stack the OLED light emitting elements on top of each other thereby allowing the areas of the light emitting elements to be larger. This approach is described in U.S. Pat. No. 5,703,436 issued Dec. 30, 1997 to Forrest et al., and U.S. Pat. No. 6,274,980 issued Aug. 14, 2001 to Burrows et al. Stacked OLEDs utilize a stack of pixel elements located one above another over a substrate. Each pixel element is individually controlled using conventional controllers. Power is supplied to the pixel elements from the controller through transparent electrodes which may be shared between pixel elements adjacent to each other in the stack.
It is also known that different OLED materials for emitting different colors of light age at different rates as they are used. It is also known to provide an OLED display having pixels with differently sized red, green and blue light emitting elements, wherein the relative sizes of the elements in a pixel are selected according to their relative aging characteristics to extend the service life of the display. See U.S. Pat. No. 6,366,025 B1, issued Apr. 2, 2002 to Yamada.
While power efficiency in a display device is always desirable, it is particularly desirable in portable applications because an inefficient display limits the time the device can be used before the power source is recharged. In fact, for certain applications the rate of power consumption may be more important than any other display characteristic including aging and resolution.
There is a need, therefore, for an improved full color flat panel OLED display having improved power efficiency.